The concept of keeping endodontically treated root fragments within the jaws to maintain bone and support dentures has become widely accepted. These dentures have been called overdentures. The primary disadvantages of the overdenture technique are that the root fragments are susceptible to periodontal disease and caries. Preventive techniques such as the home use of topical fluorides seem to be beneficial. Feagin (1983) has suggested that the mechanism of the action of F- in enamel compared to root surfaces may be different and that the commercially available fluorides may not be appropriate for use on root surfaces as they were developed for use on enamel. Feagin has developed a fluoride gel which contains 20 mM calcium ion, 10 mM phosphate ion and 0.75 mM fluoride which in vitro seems to be more effective on root surfaces. A subsample of 180 overdenture patients who have been in the study for at least one year and have shown a susceptibility to caries will be selected. These 50 patients will be randomly divided into 2 groups, one group will remain on the commercial topical fluoride gel, the other will use the experimental gel. The patients will be examined by an independent examiner every 3 months for the next 3 years. Statistical evaluation will be carried out by analysis of co-variance.